Ehrlichia chaffeensis is a tick-transmitted obligately intracellular bacterium that causes human monocytrotropic ehrlichiosis (HME), an emerging life-threatening disease in humans and also causes mild to severe disease in wild and domestic canids. (Paddock and Childs, 2003). The genomes of E. canis and other organisms in the genus, including E. chaffeensis and E. ruminantium, exhibit a high degree of genomic synteny, paralogous protein families, a large proportion of proteins with transmembrane helices and/or signal sequences, and a unique serine-threonine bias associated with potential for O-glycosylation and phosphorylation, and have tandem repeats and ankyrin domains in proteins associated with host-pathogen interactions (Collins et al., 2005; Dunning Hotopp et al., 2006; Frutos et al., 2006; Mavromatis et al., 2006). A small subset of the more than 900 proteins encoded by each of these genomes are recognized by antibody (Doyle et al., 2006; McBride et al., 2003; McBride et al., 2000). Several of the major immunoreactive proteins identified and molecularly characterized are serine-rich glycoproteins that are secreted. Many of these glycoproteins have tandem repeats; however, one has numerous eukaryote-like ankyrin domains (Doyle et al., 2006; McBride et al., 2003; McBride et al., 2000; Nethery et al., 2005; Singu et al., 2005; Yu et al., 2000).
Three immunoreactive proteins with tandem repeats have been identified and molecularly characterized in E. chaffeensis (gp120, gp47, and VLPT) as well as two orthologs in E. canis (gp140 and gp36, respectively). E. chaffeensis gp120 and gp47 are major immunoreactive proteins that are differentially expressed on the surface dense-cored ehrlichiae and are secreted (Doyle et al., 2006; Popov, et al. 2000). Extensive variability in the number and/or sequence of tandem repeats in the E. chaffeensis immunoreactive proteins (gp120, gp47 and VLPT) as well as E. canis gp36 is well documented (Chen et al., 1997; Doyle et al., 2006; Sumner et al., 1999). The gp120 contains two to five nearly identical serine-rich TRs with 80-amino acids each, and gp47 has carboxy-terminal serine-rich TRs that vary in number and amino acid sequence among different isolates of each species. Major antibody epitopes of both gp120 and gp47 have been mapped to these serine-rich acidic TRs. (Doyle et al., 2006; Yu et al. 1996; Yu et al. 1997). Similarly, the VLPT has three to six nonidentical serine-rich TRs (30 amino acids); however, the E. canis ortholog (gp19) lacks multiple TRs. The presence of tandem repeats in both coding and noncoding regions of the genome has been linked to an active process of expansion and reduction of ehrlichial genomes (Frutos et al., 2006) and is considered a major source of genomic change and instability (Bzymek and Lovett, 2001). The E. chaffeensis vlpt gene also exhibits variations in the number of 90-bp tandem repeats (2 to 6) and has been utilized as a molecular diagnostic target and for differentiation of isolates (Sumner et al., 1999; Yabsley et al., 2003).
Recently, a strongly acidic 19-kDa major immunoreactive protein of E. canis has been identified (gp19), having the same relative chromosomal location and substantial homology in a C-terminal cysteine-tyrosine-rich domain as previously reported for VLPT protein in E. chaffeensis. However, while E. chaffeensis VLPT is immunoreactive, little is known regarding its cellular location, function and role in development of protective immunity. The molecular mass of native VLPT is also unknown. It has been suggested that E. chaffeensis Arkansas strain was 44-kDa, but immunoreactive proteins consistent with that mass have not been reported (Sumner et al. 1999). The VLPT of E. chaffeensis Arkansas is a 198 amino acid protein that has four repeats (30 amino acids) and has a molecular mass approximately double that predicted by its amino acid sequence (Sumner et al., 1999). E. chaffeensis VLPT protein appears to have posttranslational modification consistent with other described ehrlichial glycoproteins, but the presence of carbohydrate on VLPT has not been demonstrated.
Defining the molecular characteristics of ehrlichial immunodeterminants involved in elicitng humoral immunity during infection is important for understanding the basis of immunity to Ehrlichia species. The present invention fulfills a need in the art by providing novel methods and compositions concerning erhlichial infections in mammals, and in particular provides methods and compositions utilizing E. chaffeensis VLPT.